1. Field of the Invention
The present invention relates to a microcomputer with a RAM and a ROM.
2. Description of the Prior Art
Conventionally, in the development of plural models of microcomputers with different memory capacities, it is general practice to start designing the chip layout for each memory capacity at the beginning stage of work. On this account, an appreciable number of design engineers are required to roll out a product line of microcomputers with different memory capacities.
Furthermore, since characteristics of CPUs and peripheral circuits may sometimes differ a little due to a difference in the wire capacitance by different chip layouts, such characteristic differences need to be taken into account in the development of microcomputers.
In view of the above, there is now under study a method of developing plural models of microcomputers of different memory capacities from a common chip layout. FIGS. 8(A) and 8(B) show examples of memory maps in the case of using a chip of a 32-Kbyte ROM capacity as a chip of a 24-Kbyte ROM capacity, FIG. 8(A) being a memory map of the chip with a 32-Kbyte ROM capacity and FIG. 8(B) a memory map of the chip with a 24-Kbyte ROM capacity. In FIGS. 8(A) and 8(B), reference character D1 denotes a memory area for a register of a peripheral device, D2 a memory area for a RAM, D3 a memory area for a flash memory or similar external memory, D4 a memory area for a ROM and D5 an area that is not used.
Conventional microcomputers are configured as described above. For example, in the case of fabricating a chip of the 32-Kbyte ROM capacity as a chip of the 24 Kbyte ROM capacity as shown in FIGS. 8(A) and 8(B), the memory area D5 from an address "8000h" to "A000h" cannot be used because other hardware structures remain the same as in the case of the 32 Kbyte memory capacity; hence, the memory area D5 goes to waste.